1. Field of the Invention
The present invention relates to a telephoto lens system that features an aperture ratio (F.sub.NO) of 1:2.8 and a half viewing angle of the order of 9.degree.. In particular, the present invention relates to an improved method of achieving focusing with this telephoto lens system.
2. Background of the Invention
Focusing with photographic lenses is generally achieved by displacing the overall lens system. However, in order to provide improved operability, lens systems that achieve focusing by displacing only part of the system have been proposed (see, for example, Unexamined Published Japanese Patent Application Nos. 65820/1984, 65821/1984, 169117/1984 and 116709/1984). In this focusing system, the overall focal length is decreased as the object to be imaged approaches the lens system so that, for a given object-to-image distance, the image magnification that can be attained is decreased as compared with the case where focusing is achieved by displacement of the overall system. In order to attain a magnification comparable to the overall-displacement case, a sufficiently large space must be provided to allow for an increased displacement of the moving components but then, size reduction of the system becomes more difficult to achieve than in the case where the overall system is displaced to effect focusing. If the displacement of the moving components is minimized, a complicated lens arrangement becomes necessary to effect compensation for the aberrations that occur where the object-to-image distance is reduced.
For the reasons stated above, the highest magnification that can be achieved by the conventional systems are within the range of from 1/10 to 1/6.